Process and means for separation of a well production stream

ABSTRACT

The invention relates to a system for separating oil/water/gas from a marginal field by using a combination of a coarse separator with short treatment duration which separates into three flows where one obtains an oil concentration of over 70%, a water concentration of over 70% and a gas quality for burning off, where the oil and water flows are treated in cyclone systems having defined geometric proportions and a gas portion is reinjected into the water injection flows without the use of a compressor. Gas/liquid cyclones may also be used before the choke valve so that the gas pressure is kept, or optionally reinjected or used for injector pump operation of low pressure wells.

FIELD OF THE INVENTION

The invention relates to a method for separating a well production flowconsisting of a mixture of oil, water and gas, where oil/water/gas aresubjected to a static separation process and are split into a light anda heavy phase, ie, a gas phase and a liquid phase.

BACKGROUND OF THE INVENTION

The invention also relates to an apparatus for separating a wellproduction flow consisting of a mixture of oil, water and gas,comprising a static separator for separating a well production flow intoa light and a heavy phase, ie, a gas phase and a liquid phase.

In connection with the growing need to recover oil from so-calledmarginal fields, there has been an increase in the need for light,compact, flexible and cost-effective processing equipment.

Many of today's known marginal fields would not be capable of providinga proper yield without the costs of the processing plants being reducedfrom today's level. A typical processing plant for an oil field consistsof oil/water separation and stabilisation, water purification, a waterinjection unit and a gas reinjection unit.

As a rule, production from marginal fields will be carried out fromships or smaller vessels which are equipped with the most essentialprocessing systems. It is therefore of great importance that theprocessing equipment also functions satisfactorily during quite largemovements, eg, during surging, heaving and tilting.

Today, oil/water separation from an oil reservoir is carried out inlarge static gravitation separators where the separation takes placeunder the force of gravity. Drawing off takes place through respectiveoverflows and underflows having great differences in height so that itis simple to draw off a pure phase which has been separated.

Static gravitation separators are large and heavy and not veryserviceable for producing from marginal fields at great depths of water,from floating platforms or vessels in motion, from the seabed ordirectly in the reservoir. There is therefore a need for a reduction inthe physical dimensions of the units, and it is thus one of theobjectives of the present invention to propose measures which allow thephysical dimensions of separation units to be greatly reduced.

SUMMARY OF THE INVENTION

According to the invention, a method is therefore proposed as mentionedabove, said method being characterised in that the liquid phase, in adehydration step, is subjected to a dynamic separation process and issplit into a light and a heavy phase, ie, an oil phase and a waterphase, and that the water phase, in a de-oiling step, is subjected to adynamic separation process in order to produce pure, produced water andreject.

Similarly, according to the invention, an apparatus is proposed asmentioned above, said apparatus being characterised by a dehydrationstep after the static separator and comprising at least one cyclone forsplitting the liquid phase into a light and a heavy phase, ie, an oilphase and a water phase, and a de-oiling step after the static separatorand comprising at least one cyclone for producing pure, produced waterand reject.

Today's established technology in the field of liquid/liquid cyclonesconsists of,cyclones having countercurrent spin, ie, the light phaseflows countercurrent to the top reject and the heavy phase passes out inthe bottom reject. One area of application for such cyclones is topurify produced water where the light phase (oil) is a small volumeflow, maximum 5,000 ppm oil (0.5%), compared with the heavy phase(water), which is 99.5% of the volume flow.

If the portion of the light phase is increased in a countercurrentcyclone, the outlets will be crucial. A choice must be made as to whichphase is to be given priority.

One of the specific aims of the present invention is to utilise theeffect of the use of countercurrent and cocurrent spin in the cyclones.By varying the cocurrent spin portion, one can vary the duration andseparation of the light phase so that the portion of the heavy phaseincreases along the axis of the cyclone so that "critical concentration"is passed (can vary from 5 to 90%) and the system goes over to a definedwater-continuous system (the heavy phase is water). This gives improvedseparation properties and the possibility of obtaining a pure light anda pure heavy phase.

On separation in cocurrent flow, the rotation of great concentrations ofthe light phase with the heavy phase is achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention shall now be described in more detail with reference tothe drawings, wherein:

FIG. 1 is a basic diagram of the invention;

FIG. 2 is a modified process diagram;

FIG. 3 is a more detailed flow chart for the implementation of themethod according to the invention;

FIG. 4 illustrates a two-phase pump with accompanying ejector;

FIG. 5 shows a modified embodiment of the pump arrangement in FIG. 4;and

FIGS. 6-10 are purely schematic illustrations of various cycloneembodiments.

DETAILED DESCRIPTION OF THE INVENTION

The function of the units which are shown schematically in FIG. 1 is toseparate oil, water and gas which flow up from an oil reservoir. Thecompletely processed oil and the water will satisfy current requirementsfor stabilised oil intended for sale and for produced water that is tobe used for reinjection or emptied into the sea. The gas is used as fuelgas or for reinjection.

In FIG. 1, a well production flow 1 comes from an oil reservoir (notshown) and passes into a static separator in the form of a coarseseparator tank 2. The separator 2 is in this case designed as athree-phase separator. The incoming flow 1 consists of oil, water andgas. This flow is separated in the separator 2 into a gas portion whichpasses out through a gas line 3 from the top of the separator, a flow 4which primarily consists of oil, and a flow 5 which primarily consistsof water.

The flow 4 consisting primarily of oil is processed further in anoil/water/gas separation module 6. The process therein is based on theuse of hydrocyclones 7 in order to be able to achieve an effectiveseparation. The use of hydrocyclones also results in it being possiblefor the separation equipment to be made far smaller than conventionalequipment for the same process. Stabilised, water-free oil passes fromthe module in a flow 8. Released gas will pass (not shown) to the flareor fuel gas system, whilst water will pass as a flow 9, for furthertreatment in a produced water module 10. The module 10 also receives aflow 5, which primarily consists of water from the separator 2. Thewater in the flow 9 contains too much oil to be capable of beingconducted straight into the sea.

The process in the module 10 is also based on the use of hydrocyclones11. Residual oil which is separated from the water is returned (notshown) to the oil/water/gas separation module 6, whilst the purifiedwater, which now contains less than 40 mg oil/liter water is emptiedinto the sea or is reinjected in to the reservoir as a flow 12.

The flow 3, which consists of gas from the separator 2 is conducted to awater injection module 13. In this module 13, the gas (flow 3) is mixedwith seawater 14 and/or produced water 15. The seawater is deoxygenatedby means of a process which, for example, is taught and described inNorwegian Patent Publication No. 160805, or another type ofdeoxygenation process. Injection water passes out as a flow 16 from themodule 13.

FIG. 2 shows a modified embodiment of the system in FIG. 1, where infront of the separator 2 there is provided an ejector cyclone systemwhich receives a well production flow from a well having higher pressureand one (or more) wells having medium or lower pressure.

The production flow from a well having lower pressure is designated 17and passes via a choke valve 18 to an ejector 19. A production flow 20comes from a well of higher pressure and passes to a hydrocyclone 21. Asshown, the top reject 22 passes to the ejector 19. From the ejector 19,a flow 23 passes to the separator 2. The bottom reject 24 from thecyclone 21 passes via a choke valve 25 to the separator 2.

From the top reject flow 22, a flow 26 branches off to an ejector 27.The gas flow from the separator 2 passes to the ejector 27 and from theejector the gas flow passes on as shown at 3. A bypass flow 28 for thegas from the separator 2 is shown.

In other respects, the system in FIG. 2 corresponds to the system shownin FIG. 1 and described above.

Thus, in FIG. 2 the flow 3 is a combination of the flow 26 from thecyclone 21 and gas from the separator 2.

FIG. 3 shows a more detailed flow chart. With reference to FIG. 1, thesame reference numerals are used for the components which are also foundin FIG. 1.

In FIG. 3, a production flow 1 passes to the separator 2. The flow 4,which primarily contains oil, passes to a cyclone 7 which forms a partof the module 6 shown in FIG. 1. The bottom reject from the cyclone 7passes as a flow 9 to a cyclone 11, which forms a part of the module 10shown in FIG. 1. The top reject 8 from the cyclone 7 passes to adehydration cyclone 30, which also belongs to the module 6 in FIG. 1.The top reject from the dehydration cyclone 30 passes as a flow 31 to atank 32 where gas is separated at the top and passes as a flow 33, eg,to a flare burner. Oil passes as a flow 34 from the tank 32 to a store.

The top reject from the cyclone 11, ie, the water flow 12 passestogether with the bottom reject 52 from the dehydration cyclone 30 to ade-oiling cyclone 35. From the de-oiling cyclone 35, the bottom reject(water) 36 passes to a tank 37 whence a gas flow 38 will flow, forexample, to a flare burner, a water flow 39 to the sea, and anoil-polluted flow 40 to a closed drain. As shown, the top reject 41(oil) passes together with the polluted flow 40.

Water from the tank 32 can, as shown, pass to the cyclone 11 by means ofa pump 53. The top reject 42 from the cyclone 11 passes as shown to theflow 31 (top reject from the dehydration cyclone 30).

The tank 32 is a so-called surge tank. The tank 37 is a degassing tankfor the produced water which comes as a flow 36 from the de-oilingcyclone 35. Reference numeral 45 denotes a heating device. The flow ofproduced water 39 from the degassing tank 37 may, as mentioned, passinto the sea, but may also be used for water injection.

Reference shall now be made once more in particular to FIG. 1, as theconditions around the gas flow 3, ie, its further processing after theseparator 2, shall now be explained in detail.

The flow 3, which consists of gas from the separator 2 (and/or of gasfrom the cyclone 21 in FIG. 2) may pass straight to a flare burner or toa water reinjection system, ie, to the module 13. In this module 13, gasis mixed with seawater 14 and/or produced water 15. Gas and producedwater are mixed and pumped further at high pressure by means of atwo-phase pump 46, as a flow 16. This flow 16 may be injected into theoil reservoir in order to sustain the pressure, which will help toincrease the recovery rate of the field. At the same time, the emissionof gas into the atmosphere will be reduced.

A conventional system for reinjection of gas will normally consist ofseveral steps including compressors, coolers and separators (scrubbers).This is a complex solution which requires major investments and a largespace.

With the invention as described below, it is possible to reduce theinvestments and space requirements considerably.

To gain best possible utilisation of the increase in pressure on theinjection of gas into the water injection flow, at the same time as thegas is to be dispersed to gain best possible pump conditions, it is ofadvantage to use an ejector 47 as shown in FIG. 4. FIG. 5 shows a secondembodiment of an arrangement having an ejector 48. Gas is conducted inas shown by means of the arrow 49 in FIG. 4 and the arrow 50 in FIG. 5.In FIG. 4, the gas is an ejector drive medium at high gas pressure inthe separator 2, whilst the suction pressure goes against the supplypressure from the pump 46. The solution in FIG. 5 may be suitable whenthe gas pressure is low. In FIG. 5, the gas 50 is drawn into the waterinjection flow and is distributed in small gas bubbles. The drivepressure is thus the water from the pump 46.

The ratio of gas for burning off to gas for the water injection flow canbe regulated. This will make possible simultaneous water/gas injectionor alternating water and gas injection, which in turn will allow theestablishment of gas/water fronts in the reservoir in order to increasethe efficiency of the water injection. At the same time, flaring isreduced, which may be an absolute necessity for the grant of permissionto start production.

The cyclones used are vital components of the system. In FIG. 6, acyclone is shown which may be called a bulk cyclone and which can beused as cyclone 7, see FIG. 3. The cyclone shown in FIG. 6 is a combinedcocurrent and countercurrent cyclone having geometric proportions asfollows:

d₀ /d₂ =0.1-1.0; l₀ /d₂ =3-11; swirl-figure Sw=μ.d₁ ×d₂ /4;

A₁ =3-15 and the countercurrent spin portion l₁ =(0-3)d₁. A₁ is the sumof all inflow cross-sections 90⁰ to the flow direction.

FIG. 7 shows a cyclone which is highly suitable as a dehydration cyclone30, see FIG. 3. The cyclone shown in FIG. 7 is primarily a cocurrentflow cyclone with the following geometric proportions:

d₀ /d₂ =0.1-1.0; conical length l/d₂ =(1-5); cylindrical length=(1-3)d₁; Sw=1-30; central oil outflow in bottom d₃ =d₁ /1.2-d₁ /10; tangentialwater outflow in the bottom d_(u) =D₁ /1.05-d₁ /4; and countercurrentspin portion l₁ /d₁ =0-2.

As mentioned, the coarsely separated water flow 5, which contains morethan 70% water, is conducted to a produced water treatment module 10. Asis illustrated in FIG. 3, this module includes both bulk cyclones 11 andde-oiling cyclones 35.

The bulk cyclone 11 shown in FIG. 3 may have the same form as thatdescribed above in connection with FIG. 6. The de-oiling cyclone 35 may,for example, be as in FIG. 8, in the form of a substantiallycountercurrent spin cyclone which makes use of a flotation effect orcoalescence effect, ie, microbubbles which arise on a drop in pressurein a saturated gas/liquid mixture. Microbubbles collide with small oildroplets and form strong bonds (oil membrane on the gas bubble) havinglow density and capable of being readily separated. The geometricproportions may be:

d₀ =d₂ =0.05-0.5, l/d₂ =3-15 and Sw=12-30, having a counter spin portionl₁ /d₁ =0-2.

The cyclone 21 used before the separator in FIG. 2 may, for example, bea cyclone as shown in FIG. 9, which separates two phases gas/liquid witha slug catcher volume 50 to even out the flow on further to a bulkseparator step or to the coarse separator 2. The advantages gained byusing this cyclone, which is shown in FIG. 2, for separation is that itcan be used prior to any form of pressure reduction, so that the gaspressure can be sustained for reinjection into the water injection flowor as driving pressure for low pressure wells. The geometric proportionsof the cyclone shown in FIG. 9 may, for example, be:

d₀ /d₂ =0.05.0.5, l/d₂ =1-5, Sw=1-30, with a countercurrent spin portionl₁ /d₁ =0-4.

FIG. 10 illustrates a typical three-phase bulk separation cyclone wherecountercurrent spin is achieved by a central annular outlet d₃ aroundthe core pipe d₀. The cyclone in FIG. 10 thus has combined cocurrentspin and countercurrent spin. For the cyclones it is the case thatcocurrent spin is achieved by a tangential inflow A₁ uppermost at thegreatest diameter of the cyclone and a central draw-off pipe d₀. Thewater phase circulates in cocurrent flow with the oil droplets so thatan extended period in the separation zone is achieved. In the space l₁from the top, the thickened/pure oil phase is drawn off through thecentral core pipe d₀. The heavy phase passes out through the bottomoutflow. In FIG. 7, the cyclone has a bottom outflow where the oil coreis drawn into a central outflow 51. In a water outflow around thecentral outflow 51 there is a tangential water ring having diameter d₄.

One of the advantages of the invention is that the whole system can bedimensioned for well pressure. As a consequence of the use of cyclonesand shorter period in the coarse separators, the pressure tanks in thesystem will require a smaller diameter and thus walls of a smallerthickness. The gas flare systems may be reduced significantly and theliquid will be subjected to lesser shearing stress throughout thepressure reduction so that the separation properties are improved.

What is claimed is:
 1. In a method for separating a well production flowconsisting of a mixture of oil, water, and gas where the oil/water/gasmixture is subjected to a static separation process and are split into alight gas phase and a heavy liquid phase, the improvementcomprisingsubjecting the liquid phase in a dehydration step to a dynamicseparation process and splitting the liquid phase into a light oil phaseand a heavy water phase and subjecting the water phase in a deoilingstep to a dynamic separation process in order to produce pure producedwater and reject, and injecting the gas phase into a water injectionflow by a booster or injector pump.
 2. A method as disclosed in claim 1further comprising splitting the liquid phase from the static separationprocess in the dehydration step into an oil continuous phase having anoil concentration of 70% or more and a water continuous phase having awater concentration of 70% or more.
 3. A method as disclosed in claim 1further comprising separating the liquid phase in the dynamic separationprocess of the dehydration step by a cyclone using combined co-currentand counter-current spin.
 4. A method as disclosed in claim 1 furthercomprising separating the water phase in the dynamic separation processof the deoiling step by a cyclone using combined co-current andcounter-current spin.
 5. A method as disclosed in claim 1 furthercomprising recycling the reject to the dynamic separation process of theliquid phase deoiling step.
 6. A method as disclosed in claim 1 furthercomprising injecting the gas phase into a water injection flow whichcontains produced water.
 7. A method as disclosed in claim 1 furthercomprising operating the method at well pressure.
 8. A method asdisclosed in claim 1 further comprising separating the well productionflow in a dynamic separation process before the static separationprocess.
 9. A method as disclosed in claim 7 further comprisingsubjecting a well production flow from a high pressure well to a dynamicseparation process, conducting the top reject therefrom to an ejectorfor actuating a well production flow having a lower pressure, andfurther subjecting the combined well production flow from the ejector tothe static separation process.
 10. In an apparatus for separation of awell production flow (1) of a mixture of oil, water and gas, comprisinga static separator (2) for separating the well production flow into alight gas phase and a heavy liquid phase, the improvement comprisingadehydration stage (6) after the static separator (2), which dehydrationstage comprises at least one cyclone (7) for splitting the liquid phaseinto a light oil phase and a heavy water phase, a de-oiling stage (10)after the static separator and comprising at least one cyclone (11) forproducing pure, produced water (12) and reject, and a booster orinjector pump (46) for infecting the gas chase into a water injectionflow.
 11. An apparatus as disclosed in claim 10 wherein the at least onecyclone in the dehydration stage is a cyclone which functions usingcombined co-current and counter-current spin.
 12. An apparatus asdisclosed in claim 10 wherein the at least one cyclone in the deoilingstage is a cyclone which functions using combined co-current andcounter-current spin.
 13. An apparatus as disclosed in claim 10 whereinthe at least one cyclone of the dehydrating stage and the deoiling stagehave a tangential inflow uppermost in the greatest diameter of the atleast one cyclone, a central upper draw-off pipe and a bottom outflow.14. An apparatus as disclosed in 13 wherein an annular upper outflow isaround the central upper draw-off pipe.
 15. An apparatus as disclosed inclaim 13 wherein a drainage pipe is outside the bottom outflow.
 16. Anapparatus as disclosed in claim 10, characterised by cyclones withratios:

    diameter/diameter

    ratio:d.sub.0 /d.sub.2

    d.sub.0 =diameter top reject

    d.sub.2 =diameter in the space 2×d.sub.1 from inflow

    d.sub.1 =diameter at inflow

    length/diameter

    ratio: l/d.sub.2

    l=length of cyclone from inflow

    A.sub.1 =sum of all inflow cross-sections 90.sup.0 to flow direction ##EQU1## having a shape where the conical part is a continuous function formed by circles, exponential or sinus functions where the actual funnel shape of the cyclone is a continuous form where the conical portion is made up of an upper portion where

    f.sub.1 (x)=A. 58.5+13.5. B. Cos(2μx/193.83. C), or

    f.sub.1 (x)=A. 72-26.6. B. Sin(2μx/393. C)

    A=0.1-176

    B=0.3-355

    C=0.1-3

and a lower portion where

    f.sub.2 (x)=2200. A(246. B+X).sup.-2/3-C

    A=0.01-499

    B=0.3-750

    C=0.1-8.